In the two-stage model of cutaneous carcinogenesis, a single subtumorigenic exposure to a carcinogen (initiation) and subsequent chronic regenerative epidermal hyperplasia of sufficient magnitude (promotion) can induce benign and malignant neoplasms. Tumor initiation is thought to convert some of the keratinocytes into latent neoplastic cells; promotion elicits expression of the neoplastic change. The identity of the latent neoplastic cell has long been a mystery. Compelling evidence has led to the hypothesis that follicular keratin 15 (K 15)-expressing keratinocytes are the cutaneous multipotential progenitors and are the target cells in two-stage carcinogenesis. Our objective is to test this hypothesis and to determine whether these cells have other characteristics of stem cells. To this end, we have developed two novel and exciting approaches. Our specific aims are as follows. 1. Identify isolate, and determine the stem cell properties of live K15-expressing cells by using the K15 promoter sequence to express green fluorescent protein in transgenic mice. 2. Determine whether K15-expressing cells are the multipotential progenitors of the cutaneous epithelium by employing a cell specific and temporally inducible strategy for fate mapping the K15-expressing cells, thereby enabling the lineage analysis of cellular replacement in the cutaneous epithelium of adult transgenic mice. 3. Determine whether the K15-expressing cells are the latent neoplastic cells in carcinogen-exposed skin by using the fate mapping strategy of Specific Aim 2 to induce the recombinase transiently in the K15-expressing cells at the time of tumor initiation, thus following the development of tumors from the presumptive progenitors. Identification and physical isolation of the stem cells in the hair follicles opens the door to health-related research of great significance including: gene therapy, the role of the stem cells in hair growth and wound healing, repair of carcinogen-DNA adducts, identification of stem cell regulatory genes, and manipulation of stem cell self-renewal. Furthermore, our novel strategy for inducible lineage analysis has applications beyond the K15 promoter or even other keratin promoters. The method will work with any promoter expressed by any progenitor cell in any organ system, thus enabling the exploration of a vast number of lineage relationships in developmental biology and in cancer research.